Micro-spectrometers with pivotable diffraction gratings have been described, for example, by H. Gruger et al. in “Performance and Applications of a spectrometer with micromachined scanning grating”; Micromachining and Microfabrication, part of SPIE Photonic West (2003).
Very small micromechanical systems are desired for a number of applications such as for scanning Accordingly, the diffraction gratings described in this disclosure are provided in correspondingly small form. The diffraction gratings are pivoted about an axis of rotation and the electromagnetic radiation which is directed onto such a diffraction grating from a corresponding radiation source is guided sequentially in a spectral range via one or more detectors suitable for the detection of specific wavelengths of the electromagnetic radiation.
Usually, highly precise and efficient micro-optical diffraction gratings are manufactured by a casting process from a so-called “master” or by holographic processes. For the manufacture from a master, the master must be created in advance. The creation of the master takes place such that equidistant lines are formed in a substrate, which consists, for example, of metal, by means of a scoring tool. The manufacture of the micro-optical diffraction gratings from such a master can then, for example, take place by means of a hardening plastic, for example, from epoxy resin. Subsequent to the manufacture of the micro-optical diffraction grating, a metallic layer of high reflectance can be applied to a surface of such a manufactured structure.
It is, however, problematic in this connection that a considerable mechanical pressure is required for the forming of the micro-optical diffraction grating and that considerable compressive forces act on the substrates which typically have a thickness of only some few 10 μm. Problems moreover occur with the required lateral adjustment precision.
In addition, the possible number of pieces of individual diffraction grating elements from such a manufacture of the micro-optical diffraction grating from the master is limited. The production costs of such micro-optical diffraction gratings suitable for micromechanical applications are naturally thereby increased.
The holographic processes for the manufacture of the micro-optical diffraction gratings are based on the interference principle by use of laser radiation. An intensity profile arises by interference of partial laser rays to produce an interference pattern. The intensity profile is substantially sinusoidal and with which a photosensitive layer on a substrate is illuminated with the interference pattern. This interference pattern (intensity profile) is then transferred onto the photosensitive layer in topological form after exposure and subsequent development. The photosensitive layer can subsequently be coated with a highly reflective metal film.
However, manufacturing apparatus, such as is usually used in a mature form in semiconductor manufacture, cannot be used for the manufacture of the micro-optical diffraction grating of this disclosure, so that an additional device in such a manufacturing apparatus is required.
It is moreover known to manufacture the micro-optical diffraction gratings with a corresponding surface topology by known process techniques of gray-scale lithography. In this connection, however, the number and/or spacing of the individual lines of the micro-optical diffraction grating is limited, so that the spectral resolution of the micro-optical diffraction grating is likewise limited.
The micro-optical diffraction gratings can, however, also be manufactured by a simple structuring of a reflective layer applied to the substrate. In this connection, a rectangular micro-optical diffraction grating can be obtained to a first approximation. The micro-optical diffraction gratings manufactured in this way, however, have a low effectiveness and can accordingly only be used for spectral analysis with high-intensity sources of electromagnetic radiation.
The manufacture of structural elements on a substrate for other purposes and in different sizes is generally known. U.S. Pat. No. 6,424,436 B1, for example, describes a holographic element which comprises a corrugated surface configuration with grooves and ridges of moderate depth and height corresponding to the pitch of the corrugated surface configuration. The holographic element is for switching light paths in dependence on the polarization directions of an incident light and comprises a plurality of regions formed substantially periodically on a substrate, by which regions the transmitted light has different phase differences between its polarization components as a whole. The pitches are less than or equal to the wavelength of the incident light. Each region, in which the corrugated surface is formed, is coated by a multilayer film comprising a plurality of layers which are made of isotropic materials of different refractive indices and laminated in such a manner that the refractive index in the film varies periodically across the thickness thereof.
The holographic element is formed on the substrate by applying a photoresist film and exposing the photoresist film to a holographic interference for forming a fine pattern. Subsequently, the substrate is subjected to a dry etching process with the photoresist film with the pattern being used as a mask. Grooves each having a width less than or equal to a half of the wavelength of a light to be incident upon the finished holographic element are formed in the substrate at a pitch equal or less than or equal to a half of the wavelength of the light.
US 2002/0098257 A1 describes the manufacture of a reflection type liquid crystal display device that displays an image by reflecting external light instead of using a backlight. Liquid crystal driving elements such as TFT are formed on the surface of a substrate. As all of the liquid crystal driving elements form a matrix of pixels. The liquid crystal driving elements are usually arranged equidistantly and in parallel to each other. The manufacturing process comprises the steps of preparing a cylindrical die unit, an outer circumferential surface of which is formed with a micro-asperity pattern. The substrate is coated with a resin film. The micro-asperity pattern is formed on the resin by pressing the outer circumferential surface of the die unit against the resin film with pressurizing means while rolling the die unit on the thin resin film. The resin film may be made of thermoplastic material and a heating unit for heating the die unit. The temperature of the resin film is kept lower than its heat decomposition temperature. By heating the resin film, the modulus of elasticity of the thin resin film can be decreased and its flowability can be increased. Thus the loads necessary for the processing such as the pressure are decreased, which makes it possible to manufacture an optical device having an accurate micro-asperity pattern.
The heating taught in US '257 serves therefore the sole purpose of easing the pressure of an embossment roll that is pressed against the thin resin film. However, the heating by itself does not cause any change of the resin film surface. The micro-asperity ultimately is caused by the surface of the embossment roller only and is not influenced by the liquid crystal elements. As can be seen for example from FIG. 16 of US '257 the grooves and pitches of the micro-asperity resin film are not in line with the liquid crystal driving elements.